The great GNU Bourne-Again Shell
Bash is the shell, or command language interpreter, that will appear in the GNU Operating System.
Bash is an sh-compatible shell that incorporates useful features from the Korn shell (ksh) and C shell (csh).
Bash Links
Useful Keyboard combinations in Bash
- ^a = start of line
Operators
Assignment Operators
=
All-purpose assignment operator, which works for both arithmetic and string assignments.Do not confuse the "=" assignment operator with the = test operator.
Arithmetic Operators
+
plus
-
minus
*
multiplication
/
division
**
exponentiation
%
modulo, or mod (returns the remainder of an integer division operation)
+=
"plus-equal" (increment variable by a constant). let "var += 5" results in var being incremented by 5.
-=
"minus-equal" (decrement variable by a constant)
*=
"times-equal" (multiply variable by a constant). let "var *= 4" results in var being multiplied by 4.
/=
"slash-equal" (divide variable by a constant)
%=
"mod-equal" (remainder of dividing variable by a constant)
Logical Operators
&&
and (logical). if [ $condition1 ] && [ $condition2 ]
||
or (logical). if [ $condition1 ] || [ $condition2 ]
Dont confuse those with #Conditional executing.
Comparison Operators
Integers
-eq
is equal to. if [ "$a" -eq "$b" ]
-ne
is not equal to. if [ "$a" -ne "$b" ]
-gt
is greater than. if [ "$a" -gt "$b" ]
-ge
is greater than or equal to if. [ "$a" -ge "$b" ]
-lt
is less than. if [ "$a" -lt "$b" ]
-le
is less than or equal to. if [ "$a" -le "$b" ]
<
is less than. (within double parentheses) (("$a" < "$b"))
<=
is less than or equal to. (within double parentheses). (("$a" <= "$b"))
>
is greater than (within double parentheses). (("$a" > "$b"))
>=
is greater than or equal to (within double parentheses). (("$a" >= "$b"))
Strings
=
is equal to. if [ "$a" = "$b" ]
==
is equal to. if [ "$a" == "$b" ]
!=
is not equal to. if [ "$a" != "$b" ]
<
is less than, in ASCII alphabetical order
>
is greater than, in ASCII alphabetical order
-z
string is "null", that is, has zero length
-n
string is not "null".
Bitwise Operators
<<
bitwise left shift (multiplies by 2 for each shift position)
<<=
"left-shift-equal" let "var <<= 2" results in var left-shifted 2 bits (multiplied by 4)
>>
bitwise right shift (divides by 2 for each shift position)
>>=
"right-shift-equal" (inverse of <<=)
&
bitwise and
&=
bitwise and-equal
|
bitwise OR
|=
bitwise OR-equal
~
bitwise negate
!
bitwise NOT
^
bitwise XOR
^=
bitwise XOR-equal
Misc Operators
,
comma operator . The comma operator chains together two or more arithmetic operations. All the operations are evaluated (with possible side effects), but only the last operation is returned.
Conditional executing
command1 && command2
command2 is executed if, and only if, command1 returns an exit status of zero. (exit status zero means succesful)
command1 || command2
command2 is executed if and only if command1 returns a non-zero exit status. (non-zero exit status means failed)
command1 && command2 || command3
if command1 is executed successfully then shell will run command2 and if command1 is not successful then command3 is executed.
Wildcards / regex
These can be used on filenames.
*
zero or more characters
?
exactly one character
[abcde]
exactly one character listed
[a-e]
exactly one character in the given range
[!abcde]
any character that is not listed
[!a-e]
any character that is not in the given range
{debian,linux}
exactly one entire word in the options given
You can use wildcards with any command that accepts file names as arguments.
also see: Bash Scripts